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Old 12-23-2017, 04:48 PM   #21
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Sounds like that little wreck off manalapan.
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Old 12-23-2017, 09:09 PM   #22
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Sounds like that little wreck off manalapan.
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Old 12-23-2017, 09:37 PM   #23
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I also like my paper charts. I found that the Nobletec Time Zero app on my IPad gives me all raster (paper charts) for North America for $50. Hard to beat that. although I still can’t get rid of the actual paper.
Does this include Canadian charts? I know the US charts have largely been made available to print yourself for free, but I still see a minimum of over $300 for any bundle of Canadian charts, especially the detailed ones for the BC upper coast.
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Old 12-24-2017, 06:03 AM   #24
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Does this include Canadian charts? I know the US charts have largely been made available to print yourself for free, but I still see a minimum of over $300 for any bundle of Canadian charts, especially the detailed ones for the BC upper coast.
I mis spoke on that. The charts for USA are $49.95 and Canada is an additional $49.95. (Canadian covers East coast, West coast, and Great Lakes.) I tried but cannot correct my previous post.

For clarification you can’t print the charts from Nobletec (I don’t think?) but they are all available to be downloaded to your IPad as needed, once you have paid.
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Old 12-24-2017, 08:52 AM   #25
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interesting article on using sonar charts for navigation:
https://www.panbo.com/archives/2017/...cs_charts.html
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Old 12-24-2017, 01:15 PM   #26
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I have “iNavx” navigation software on my iPad together with A) NOAA Raster Charts for the PNW, B) CHS Raster Charts for British Columbia and C) Navionics “2XG Canada” Charts for the PNW. I also have Coastal Explorer with the A) NOAA Raster and ENC Charts for the PNW, B) CHS Raster and ENC Charts for British Columbia.

I’ll comment on the “Crowd Sourcing” by Navionic below, but it’s important otherwise that we realize that for any given location, ALL NAVIGATION PROGRAMS USE THE SAME SOURCE DATA, WARTS AND ALL. It doesn’t matter if you have Nobeltech’s “Wizzy” 3D representation of the seafloor, it uses no more information than what is displayed on the highest resolution raster charts for any given location. At least I know that to be true in Canadian waters. Others can comment on US waters.

So what are the warts?

It starts with how the coastline has been drawn on the raster charts; in particular locations on the coast surveyors determined the “horizontal datum” (or, the plural form “horizontal data”): i.e. “precise” determinations of the exact locations of “points” on the shore. These were marked by brass plugs inserted into the rock. They then marched around the shoreline and completed the shoreline survey by triangulation and “drew” the shoreline. Next they sent some guys out in boats, who took “leadline” measurements and they made “fixes” of the vessel in the water RELATIVE TO THE SHORELINE “DATA” LOCATIONS. If there is bias in the determined location of the Horizontal Datum, that bias will be propagated through the entire part of the chart that is based on that determination.

So, how “precise” is “precise”? It turns out sometimes “NOT VERY”. Consider the statement on CHS Raster Chart 3726 just below Higgins Passage: “Recent surveys in the general area of Price Island, and in particular Higgins Passage, indicate that horizontal position discrepancies in the order of 150 m may exist. Mariners should use caution when navigating in these areas.” So some of you might say: Those are raster charts and I use ENC’s! Well on Coastal Explorer you can switch back and forth between the two types of charts and at least for my boat, its position does not change when referenced to land features on the charts.

Now, if you use the raster charts as they were intended, none of this maters, because you’d take some of compass fixes on two or more reference points on land and you’d “triangulate” your position “relative” to those points. But with GPS, everything’s changed because your boats positon relative to the geostationary satellites is “different” than what would be “referenced” from the paper chart.

So, what about “Crowd sourcing”? I am deeply suspicious about it, at least for now. Perhaps I can be convinced otherwise. Crowd sourced information comes from people whose equipment has not been calibrated for accuracy and precision. Is the transducer’s position known with accuracy? Is its orientation exactly vertical? Have the soundings been corrected for “Pitch, Roll and Yaw”? (NB: if the soundings have not been corrected for pitch roll and yaw, the depths will be overestimated.) You might say that if you have enough “observations” you could discard the outliers from the data but as always the devil is in the details.

Enough already! I’ll close my comments with a screen capture from Coastal Explorer that I’ve posted before of the AIS target of the CHS Survey Vessel Otter Bay traveling over land at Murchison Anchorage!
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Old 12-24-2017, 02:19 PM   #27
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I have seen class B AIS targets also cross over land. I attributed that to the fact that they only broadcast their position every so often. So in theory when approaching a spit of land or an island for example they (the AIS) broadcast its position. Then they travel around the island or spit of land and the (AIS) broadcast its position again. The Track will show a straight line from the first to the second broadcast positions. Right over the island or spit of land. Plausible?
I am open for correction
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Old 12-24-2017, 03:08 PM   #28
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I have seen class B AIS targets also cross over land. I attributed that to the fact that they only broadcast their position every so often. So in theory when approaching a spit of land or an island for example they (the AIS) broadcast its position. Then they travel around the island or spit of land and the (AIS) broadcast its position again. The Track will show a straight line from the first to the second broadcast positions. Right over the island or spit of land. Plausible?
I am open for correction
A 30 second transmit cycle for Class B. I would hesitate to even call these lines "tracks". More of a visual aid to connect the same owner of the positions, than a track. A lot can happen in 30 seconds at 20kts, and if that packet is lost, you now at 60 seconds.
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Old 12-24-2017, 04:26 PM   #29
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Quote:
Originally Posted by Easting View Post
I have seen class B AIS targets also cross over land. I attributed that to the fact that they only broadcast their position every so often. So in theory when approaching a spit of land or an island for example they (the AIS) broadcast its position. Then they travel around the island or spit of land and the (AIS) broadcast its position again. The Track will show a straight line from the first to the second broadcast positions. Right over the island or spit of land. Plausible?
I am open for correction
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Originally Posted by diver dave View Post
A 30 second transmit cycle for Class B. I would hesitate to even call these lines "tracks". More of a visual aid to connect the same owner of the positions, than a track. A lot can happen in 30 seconds at 20kts, and if that packet is lost, you now at 60 seconds.
No. It's a Canadian Coast Guard Vessel that is tasked with doing the hydrographic work for the Canadian Hydrographic Service with a need for the best GPS and it will have Class "A". I watched the whole transmission and it transmitted every few seconds as it left the anchorage. I talked about it with the crew of this vessel earlier before they left and they confirmed the problem with shoreline on the chart. I also went by the spot in my skiff and verified the issue with the iNavx app on my iPad. This IS NOT AIS error, but rather horizontal position error on the charts.

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